Operator Overloading: indexing

1. Operator Overloading: indexing • Useful to create range-checked structures: class four_vect { double stor[4]; // private member, actual contents of vector. … double& operator[ ] (int j) { if (j < 0 || I > 3) throw index_error; // defined elsewhere return stor[j]; }; Note: return type is a reference, so can be used on l.h.s

2. Extending the meaning of subscripting • An associative array: class Assoc { // a map from strings to numbers struct Pair { // an inner class char* name; double val; Pair (char* n = “”, double v = 0) : name (n), val (v) { }; }; pair * contents; // Assoc is a set of pairs public: Assoc ( ) { }; // default constructor double& operator[ ] (const char*); // map string => number };

3. Efficiency vs. privacy • Linear algebra package: vector class for 4-vectors, matrix class for 4-by-4 matrices • vector class redefines [ ] to do check index • matrix class redefines [ ] to check both indices • want to implement matrix * vector: • v2 [j] = S m[j][k] * v [k]; • 4 range-checks for every component, all redundant.

4. Relaxing privacy: friends • A friend function can access private members of a class • a friend is declared in the class (cannot sneak in) class vector {... friend vector operator* (const matrix&, const vector&); • a function can be the friend of several classes class matrix {… friend vector operator*(const matrix&, const vector&); • A class can be a friend, so all its function members are. class quaternion { friend class matrix; …} • A friend is not a class member

5. Efficient code class matrix { vector row [4]; public: … }; vector operator * (const matrix&m, const vector v) { vector res; for (int i = 0; i < 4; i ++) { res.stor[i] = 0; // stor is private data in a vector for (int j = 0; j < 4; j++) res.stor[j] += m.row[i].stor[j] * v.stor[j]; // row is private }; return res; };

6. Packages and private information • In Ada, declare both types in same package. Package body has access to full declarations for both: package linear_algebra is type vector is private; type matrix is private; function “*” (m : matrix; v : vector) return vector; private type vector is array (1..4) of long_float; type matrix is array (1..4) of vector; -- code in body can use array representation of both. end linear_algebra;

7. Inheritance • General mechanism for extending existing classes. • Specialization: an X is an A • A mammal is a vertebrate • a feline is a mammal • a cat is a feline • a persian is a cat • A persian has all the attributes of a feline, a mammal, and a vertebrate • A class hierarchy implements a sequence of Is-A relations

8. Advantages of inheritance • Factorization: Common properties are grouped in a single class • code reuse : Descendant class (derived class) is defined incrementally over parent. • incrementalprogramming : New derived classes can be added without changing existing parents and siblings. • Polymorphism: Can describe collections of diverse objects belonging to a class hierarchy.

9. Derivation class point { // base class int x, y; public: point () { x = 0; y = 0}; // basic constructor void move (int dx, int dy) { x += dx; y += dy}; } void display ( ) ; // put black dot on the screen. } class color_point: public point { // derived class int R, G, B; // in addition to hidden members x, y public: color_point ( ):point ( ) {R = 50; G = 50; B = 50;}; // call parent constr. void lighten (int w) { R -= w; G -=w; B -= w;}; void display ( ); // put colored dot on the screen // move is inherited, applies to color_points as well };

10. Substitution rule • An object from a derived class can be used wherever an object from the base class is expected. point* p1 = new color_point ( ); • works because a descendant has all the attributes of the parent (possibly with different meanings). color_point* wrong = new point ( ); // error: incomplete object • but p1 can be coerced (cast) to a color_point, because it is one. • Formal definition: the Liskov substitutability principle

11. inheritance and privacy • Private members of the parent class are not visible in (the methods of) the derived class. int color_point::abcisa ( ) {return x; }; // error • Constructor for parent is used to initialize parent data. Derived (…) : Parent (…) { …}; // rest of construction • Protected members are visible in descendants, but not outside.

12. Polymorphism • Because of substitution rule, can have collection of various kinds of points: point* figure [100]; • To display collection, apply the appropriate display method to each: point::display () color_point::display () blinkling_point::display () • Could add discriminant to each object, to recognize its class • Best to use virtual methods.

13. Virtual methods class parent { virtual void change (int x) … class child: public parent { virtual void change (int x) … // overrides parent operation class grandchild: public child { virtual void change (int x)… // overrides child operation parent* someone = … // can point to any member of family someone-> change () // the proper one for someone’s class

14. Dynamic dispatching • If M is virtual, the call ptr -> m (..) must determine the actual nature of ptr, and invoke the proper method • Each class is described by a table of virtual methods (the vtable) • Each object carries a pointer to the vtable. • Each derived class inherits the table • each overriding modifies an entry in the table • Dynamic dispatching is an indirect call through an entry at a known position in the vtable.

15. Overloading and dynamic dispatching • Overloading resolution is a compile-time process that applies to an expression in a context: that = this_one + the_other; // find right numeric type • dynamic dispatching is a run-time activity that applies to a reference: some_relative-> display (); // indirect call tommy.display ();// static resolution • overloading resolution may fail if call is ambiguous • dynamic dispatching succeeds because derived classes have all the virtual methods of ancestors.

16. The profile of an overriding method class parent { virtual void method (int x); virtual void relation (parent x); class child: public parent { virtual void method (int x); // overrides inherited method virtual void method (double x); // different method Binary operations are not symmetric: virtual void relation (child x); // does not override parent relation virtual void relation (parent x); // overrides

17. Abstract classes • Base class may not have enough attributes to create meaningful object: class shape { point anchor; int color [3]; virtual void display (); // can’t write anything here }; shape blob; // can’t do anything with it • If methods are declared abstract, cannot create objects of class: virtual void display ( ) = 0; // must be overridden

18. A useful pattern: the factory • Need to create class hierarchy. • Need different versions of hierarchy on different platforms (e.g. Windows vs. Linux) • Need to minimize dependence on platform. • Solution: encapsulate “constructors” into an object: class maker { virtual class1* build_class1 (int) = 0; virtual class2* build_class2 (int) = 0; };

19. Everything can be solved with indirection class win_maker: public maker { virtual class1* build_class1 (int){ ...}; // implementation virtual class2* build_class2 (int){…}; // on windows }; class linux_maker: public maker { virtual class1* build_class1 (int){ ...}; // implementation virtual class2* build_class2 (int){…}; // on RedHat 6.2 }; maker* factory = .. // build the proper one class1* thing1 = maker -> build_class1 (5);

20. Multiple inheritance • Class can inherit from several parents: class abstract_stack { // algebraic definition public: void push (int x) = 0; ... }; class vector {…}; // sequential allocation class stack: public abstract_stack, public vector { public: void push (int x) { .. }; // use vector primitives }

21. Multiple inheritance and overloading class A { … void draw ( ); }; class B { … void draw ( ); }; class C: public A, public B { ... void display ( ) { draw ( ); // ambiguous: 2 inherited methods A:: draw ( ); // OK };

22. Replicated base classes class Root {… // some data members class A: public Root { … // inherits data members class B: public Root { … // inherits same data members class hybrid: public A, public B {… // two copies of data members of Root? class C: virtual public Root {… // single copy in children

23. Other complexities • Additional run-time structures for dispatching: • Data members are concatenation of data members of parents • Vtable is concatenation of Vtable’s of parents. • Each operation needs to compute offset to find parent data members in descendant object. • Multiple inheritance best when it is interface inheritance, or when implementation is inherited from only one parent: • Java and Ada model: one parent, many interfaces. • Interfaces are abstract classes that only have operations • “inheriting from an interface is like inheriting a mortgage”